1. Field of the Invention
The present invention relates to a leveling apparatus that measures a predetermined amount of powder, and more particularly, to a measurement method and a leveling apparatus suitable for powder used in a method for manufacturing friction material such as disc pads, brake linings, and clutch facings.
2. Description of the Related Art
Brake linings, clutch facings and the like are affixed to a brake shoe and a clutch disc, respectively, by an adhesive agent, rivets, or the like. Of these, some, like a disc pad, are made by attaching a metal plate called a back plate to the friction material by thermocompression molding.
The friction material is formed by filling a thermocompression mold with powdered friction raw material, composed of a mixture of fiber, filler, binder and the like and heating and compressing it with a press machine (hereinafter “press”). There are two such methods of forming the friction material. One method involves directly filling the thermocompression mold with the powdered friction raw material and molding it by pressing it with a press. The other method involves filling the compression mold with the powdered friction raw material and compressing it with a press without heating to form a preform. The preform is then put into a separate thermocompression mold and heated and pressed with a press. In a disc pad, the back plate overlays the powdered friction raw material and is heated and pressed so as to adhere the back plate to the friction material.
Although the above-described friction material may be a single layer composed of a single composition, it is not uncommon for the friction material to be of two- or a multi-layer construction. In disk pads in particular, in order to obtain adequate strength of adhesion and shearing strength between the back plate and the friction material, as well as both adequate heat insulation to retard the propagation of heat from the friction surface of the friction material to the back plate and adequate vibration absorption in order to prevent the occurrence of screeching, an intermediate layer composed of a material that behaves differently from the friction layer is often provided between the friction layer that acts as a brake and the back plate.
In the conventional multi-layer friction material structure having two or more layers, the weight of both the raw material for the substantially powdered friction layer and the raw material for the substantially powdered intermediate layer, respectively, is measured, and as an initial step the raw material for the intermediate layer is spread to a uniform thickness inside a compression mold, after which the raw material for the friction layer is spread atop the raw material for the intermediate layer. The layers are then compressed with a press to form a preform. Where the step of creating a preform is not taken, a thermocompression mold is filled with the friction layer raw material and the intermediate layer raw material, the back plate is directly contacted against the material and the whole is heated and compressed by the press to form a finished product.
However, with the above-described conventional method, because the powdered raw material for forming the intermediate layer is measured by weight and a given weight is either manually or mechanically spread to a uniform thickness inside the mold, it is easy for the thickness of the layer to be or become uneven. Typically, in contrast to the thickness of the friction material, which is approximately 10 mm, the thickness of the intermediate layer is approximately 1-2 mm. Given such thinness, any unevenness in the thickness of the intermediate layer can produce local deficiencies in that thickness, which in turn can give rise to insufficient adhesive strength, shearing strength, heat insulation, and vibration absorption. Consequently, conventionally, in part because it had been impossible to give the intermediate layer a uniform thickness, the intermediate layer has been made extra thick, so that, even if local thinness dose arise, there is no loss of the adhesive strength, shearing strength, heat insulation and vibration absorption properties described above.
However, such an approach, while solving the problem of the intermediate layer material only slightly, leads to the opposite problem of localized thickness of the intermediate layer, in other words, too much material. Where too thick, the intermediate layer becomes exposed extremely quickly once the friction material wears away, leading to localized deterioration in friction performance such as degradation of the coefficient of friction, wear, and fade resistance.
Thus, as described above, where the friction material consists of a multi-layer structure, it is very difficult to make the thickness of the layers uniform.
In order to overcome this drawback, Japanese Laid-Open Patent Publication No. 11-226979 proposes a friction material manufacturing apparatus like that shown in FIG. 14. That is, in FIG. 14, a slide plate 3 having a cavity 3a of a uniform depth is disposed so as to be horizontally slidable between a plunger 2 and a fixed mold 5 and a hopper 6 for intermediate layer raw material is provided near a frame 1, with an exit aperture 6a contacting the slide plate 3.
The slide plate 3 itself is disposed so as to contact a bottom edge 1b of the frame 1. A cavity 3a of the same shape as the friction material is provided in the shape of an aperture penetrating the slide plate 3, with the bottom of the hole covered by a bottom plate 4. Both the slide plate 3 and the bottom plate 4 can be moved in the horizontal direction. When the cavity 3a is positioned directly beneath the frame 1, the bottom plate 4 rests atop the fixed mold 5. A small cavity 4a corresponding to the boss of a preform is formed in the bottom plate 4. The slide plate 3 and the bottom plate 4 break apart into two parts along an imaginary line that passes through the center of the small cavity 4a. 
When the slide plate 3 moves horizontally and the cavity 3a comes under the hopper 6, the cavity 3a and the small cavity 4a are filled with intermediate layer raw material 8. When the slide plate 3 returns to its position beneath the frame 1, the surface of the bottom edge 6b of the hopper 6 becomes a leveling plate that makes the thickness of the intermediate layer raw material 8 uniform. When the cavity 3a returns to a position beneath a cavity 1a in the frame 1, a certain amount of friction layer raw material 9 enters the empty space 1a from above and is made a uniform thickness, after which the plunger 2 descends and compresses the layers of material to form a structure. By using multiple slide plates 3 and filing a different hopper with more powdered intermediate layer raw material, a multi-layered structure can be obtained.
It is desirable that as much fibrous material as possible be mixed into the intermediate layer in order to increase the strength of adhesion and shearing strength between the back plate and the friction material. However, with the apparatus described in Japanese Laid-Open Patent Publication No. 11-226979, there is a limit to the amount of fiber that can be mixed into the intermediate layer. The intermediate layer has a thickness of approximately 1 mm when formed by thermocompression and the cavity 3a has a depth of approximately only several millimeters. If a large amount of fibrous material is mixed into the intermediate layer raw material, then when leveled by the surface of the bottom edge 6b of the hopper 6 as shown in FIG. 14 the intermediate layer raw material cannot be leveled cleanly because the fibers that contact the surface of the bottom edge 6b of the hopper 6 are pulled along by the edge of the surface of the bottom edge 6b, which in turn appears to cause a substantial amount of powdered raw material to be pulled along therewith. As a result, when leveling is finished, there arises a situation in that in some areas there is absolutely no powdered raw material at all, the leveled surface is not flush, and there are bumps and depressions. If the intermediate layer is exposed to thermocompression molding in this state, the result will be an uneven intermediate layer and/or friction material with no intermediate layer backing.